Several approaches have been used in the prior art to prevent counterfeit reproduction of such documents, including intaglio printing methods such as used on various banknotes and the like, and halftone imaging.
Intaglio printing requires expensive steel engraving and substantially increases the cost of the document using this method. The use of halftone printing is inexpensive and various methods of using the technique are disclosed in U.S. Pat. Nos. 3,675,948, 4,265,469 and 4,310,180. Other methods of producing non-counterfeitable documents are disclosed in U.S. Pat. Nos. 4,176,859 (different line screen frequencies), 1,002,600 (overlaying parallel lines disposed at different angles), and 4,246,307 (laminated sheets with spaced adhesive zones to damage the sheets in the event of tampering).
The halftone printing method described in Wicker, U.S. Pat. No. 3,675,948, requires the use of multiple halftones or tints. In this patent, a concentric rosette pattern is produced. This phenomenon occurs naturally and is a usual feature of multiple color half-tone printing. However, it is not a natural feature for single color halftones or tints. Instead, this feature (a rosette pattern) in a single color security printing would be suspect, perhaps leading a counterfeiter to the method for duplicating the security feature.
The line printing method described in Morris, et al, U.S. Pat. No. 1,002,600, prints overlays with multiple lines disposed at an angle to one another in different overprints. A special screen is required for viewing the hidden image. However, the screen is not a tint and the use of the screen does not produce a "moire" effect. Moreover, the image is not totally hidden in this method with one color printing.
It is desirable to produce documents in which a security feature is provided that does not substantially increase the cost of the document and which cannot be readily reproduced by counterfeiter, and further, which does not require any special lighting or conditions to make a validity check.